1. Field of the Invention
The present invention relates to a method of measuring a plating amount and a plating film composition of a plated steel plate to be measured by an in-line scheme and a measuring apparatus therefor and, more particularly, to a method of measuring plated steel plate plating amount and plating film composition effective to analyze a portion at which a plating film contains the same component as an underlying metal and a measuring apparatus therefor.
2. Description of the Related Art
One method used to measure a plating amount or a plating film composition of a plated steel plate to be measured, is the fluorescent X-ray analyzinq method. According to this method, after X-rays are radiated onto a steel plate to be measured, the intensity of fluorescent X-rays as a function of the plating thickness and the plating film composition are measured, and the measured value is compared with a calibration curve. In the case of the steel plate having a plating film not containing an underlyinq metal, such as a Zn-plated or Zn-Ni-plated steel plate its plating amount or the plating film composition can be measured by an in-line scheme.
If, however, a plating film of a steel plate contains an underlying metal, as in the case of, for example, a Zn-Fe-alloy-plated steel plate, which has recently attracted attention because of its high corrosion resistance and good workability, measurement of the plating amount and the plating film composition by means of the fluorescent X-ray corresponding to analyzing method has the following disadvantages: Since, when using the fluorescent X-ray analyzing method, it is difficult to distinguish fluorescent X-rays F contained in a plating film from those corresponding to Fe as an underlying metal, it is difficult to obtain an interrelation between the fluorescent X-ray intensity and the plating amount and the plating film composition, and the plating amount and the plating film composition cannot be measured by an in-line scheme.
Consequently, the following two analyzing methods have been proposed with the aim of overcoming the above disadvantages.
One method proposed is an in-line analyzing method (Published Unexamined Japanese Patent Application No. 58-223047) in which so-called white X-rays having a plurality of wavelengths are radiated onto a Zn-Fe-alloy-plated steel plate, and the intensity of a K-series fluorescent X-ray is measured by detectors located at a 1st-measurement-angle position, from which substantially no fluorescent X-rays from a metal underlying the plated steel plate can be detected in terms of depth of X-ray penetration, and at a 2nd-measurement-angle position, from which the fluorescent X-rays from the underlying metal can detected, whereby the plating amount and the plating film composition can be obtained on the basis of both measured values.
The other method is one (Published Unexamined Japanese Patent Application No. 60-169533) in which the plating amount is measured on the basis of diffraction X-rays corresponding to o-Fe of an underlying metal, by way of utilizing the absorption by a plating film of a Zn-Fe-alloy-plated steel plate, and wherein the plating film composition is measured on the basis of the diffraction X-ray intensity of at least one phase selected from a Zn-Fe-alloy phase and an .eta. phase in the plating film.
However, in the case of the former of the above two fluorescent X-ray analyzing methods, the following problems arise, due to white X-rays being used as incident X-rays.
(a) Since high-energy X-rays in white X-rays do not attenuate very much in a plating film of a plated steel plate, they therefore can penetrate to a considerable depth. Consequently, the 1st measurement angle must be set to 5 or less in order to ensure that fluorescent X-rays from an underlying metal are not detected. As a result, the measurement distance varies in accordance with variation in the perpendicular direction to the surface of the plated steel plate, the fluorescent X-ray intensity also varying in accordance with variation in the measurement angle, giving rise to a problem of low measurement precision.
(b) The plating amount and the plating film composition may be obtained by comparing a measured intensity obtained by actually radiating X-rays on a plated steel plate, with a theoretical intensity obtained by a known theoretical intensity relation. Since, however, calculation of the theoretical intensity is adversely affected by spectral variation in incident X-rays, caused by, for example, deterioration over time of the X-ray tube, measurement precision is undesirably decreased.
(c) In the case of calculating an analysis value by comparing the measured intensity with the theoretical intensity calculated from the known theoretical intensity relation, the calculation time is prolonged, since wavelength integration must be performed upon calculation of the theoretical intensity. Consequently, the measurement time is increased.
(d) A calibration curve can be used in order to overcome the disadvantage described in item (c) above. Using this method, however, 20 to 30 types of standard sample must be prepared in order to form a model considering a matrix effect, resulting in a very time-consuming troublesome analyzing method.
The latter diffraction X-ray analyzing method has the following drawbacks:
(a) Since the diffraction X-ray intensity corresponding the .alpha.-Fe of an underlying metal depends on not only the plating amount but also on, for example, the type or thickness of the steel plate, the texture of the steel plate, which changes in accordance with manufacturing conditions of a plated steel plate and the like, and the plating film composition, this gives rise to a problem of measurement precision.
(b) The diffraction X-ray intensity of an alloy phase changes in accordance with plating conditions, and the structure or composition of an alloy differs depending on whether the plating material used is a molten plating material or an electric plating material. Therefore, satisfactory measurement precision can not be obtained.